As this type of the fluid control unit, as shown in the patent document 1, there is a fluid control unit that comprises a flow rate measurement part that measures a flow rate of a fluid flowing in a flow channel and a fluid control valve arranged in the flow channel, and that controls a valve aperture of the fluid control valve so as to make a measured flow rate measured by the flow rate measurement part follow a preset flow rate.
More concretely, the fluid control unit outputs a valve aperture operation amount of the fluid control valve so as to lessen a deviation between the measured flow rate and the preset flow rate by the use of the deviation and a control coefficient such as a previously set PID coefficient or the like. A case that the valve aperture operation amount is an applied voltage to the fluid control valve will be explained below.
First, considered is a stable state wherein the deviation between the measured flow rate and the preset flow rate is small so that behavior of the measured flow rate is stable. In this stable state, a gain of the above-mentioned control coefficient is set to be big so that fluctuation of the applied voltage to the deviation becomes big. This arrangement is to keep the behavior of the measured flow rate in the stable state by immediately returning the applied voltage after the noise fluctuation around the middle value, even though the applied voltage that is stable around a certain value (hereinafter called as a middle value) appropriate for control in a stable state fluctuates (hereinafter called as causing a noise fluctuation) due to an external disturbance.
On the other hand, a transitional state wherein the measured flow rate fluctuates toward the preset flow rate after it fluctuates is considered by changing the preset flow rate. In this case, if the control coefficient is big, fluctuation of the applied voltage after the preset flow rate fluctuates becomes too big so that so called undershoot or overshoot occurs. Then, the overshoot or the undershoot is restrained by setting the gain of the control coefficient in the transitional state to be small relative to that in the stable state.
However, in the above-mentioned control method, as shown in FIG. 12, in case that the noise fluctuation occurs on the applied voltage at a timing when the stable state is switched to the transitional state, since the control coefficient is so set to moderate the fluctuation of the applied voltage in the transitional state, the applied voltage does not quickly return in the transitional state after the noise fluctuation.
In accordance with this arrangement, if the noise fluctuation tends to increase, the valve aperture of the fluid control valve unexpectedly becomes big in the transitional state. Conversely, if the noise fluctuation tends to decrease, the valve aperture of the fluid control valve unexpectedly becomes small in the transitional state. As a result of this, as shown in FIG. 13, for example, a spike-shaped peak appears on the measured flow rate.